Electron discharge apparatus



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I ELECTRON DISCHARGE APPARATUS Filed April 18, 1952 2 Sheti -Sheet 1 INVENTOR TERRY M SHRADER ATTORNEY Patented Apr. 14, 1936 UNITED STATES,

ELECTRON DISCHARGE APPARATUS Terry M. Shrader, West Summit, N; 1., assignoi' to Radio Corporation of America, a corporation of Delaware Application April 18, i932, Serial No. 605.879 10 Claims. (Cl. 250-275) My invention relates to electron discharge devices or thermionic tubes and circuits for utilizing them, and more particularly to thermionic tubes having two or more grid electrodes-interposed between the cathode and the plate electrode,

and to various circuits and combinations in which such a tube may be used to advantage.

v Screen grid tubes, which have two grid electrodes interposed between the cathode and the plate electrode, and pentodes, which have a control grid and screen grid and also a third grid electrode or suppressor grid interposed between the screen grid and plate electrode and usually connected internally to the cathode, are known in the art. As usually made, both the screen grid tube and the pentode have suificient capacity between plate and ground to impose some limitations on their use for some purposes, such as with short waves having a length of about five (5) meters, and in both of them the grid electrode adjacent the cathode is used as a. control element.

One object of my invention is to provide a multiple grid tube having in general the advantages of screen grid tubes and pentodes, but so constructed that its characteristics can be varied at will to a greater extent than in the tubes heretofore used, and capable of being operated soas .to have characteristics approaching those of a "tubes heretofore used. A further object is to provide a rugged and strong multiple grid tube which has low inter-electrode capacity, and which can beinade with great accuracy by usual manufacturing methods-and equipment. Still another object of my invention is to provide various circuits and combinations to utilize the properties and advantages of a tube having three grid'electrodes between the cathode and the plate electrode and constructed in accordance with my invention. Various other features and advantages of my invention will appear from the description which follOWS.

around the cathode to be passed successively by the electron stream from the cathode to the anode. Each'oi' the electrodes has its own separate and independent terminal on the outside of the sealed envelope or bulb which encloses the electrodes, one of these terminals,iorexample the terminal for the first or control grid nearest the cathode, being mounted on the top of the bulb,

and the other-terminals on the usual base secured to the opposite-end of the bulb. The base is provided with six ,gontact pins or terminals of the usual type, oneeonnectemto the plate electrode, two to the cathode heater, one to the cathode sleeve, one to the second grid, and one to the third grid. Such a tube, in which the electrodes can be connected in many different ways, and various voltages can be applied to the electrodes independently at will, is a new tool in the art, of great flexibility and adaptability, and with which many new results can be obtained. By the application of suitable voltages to the electrodes, and particularly by operating the first and second grids as control and screen grids, and the third grid as an independent'regulator grid, preferably negative with respect to the cathode, important characteristics of the tube can be varied at will, and the flow of the electron stream from the cathode to the plate electrode can be regulated and modified more completely and in a greater variety of ways than in the screen grid tube or in the pentode. For example, by applyw. ing the proper negative voltage to the third or regulator grid the space charge, or cloud of electrons around the cathode, can in efiect be brought out to the vicinity of the second or screen grid, in which case the effect of the control grid near the cathode is decreased, and the tube has characteristics approaching those of a triode as the screen grid becomes effectively, at least to some extent, a cathode. Under these conditions the plate electrode can draw electrons thru the third grid alone easier than thru all three grids, hence the voltage gain falls on? and the plate resistance falls to a low value. With this type of control considerable improvement in the control of gain and of fidelity is possible.

The inter-electrode capacities and the plate to ground capacity are minimized in the preferred construction of the tube, in which I prefer to use a rigid and mechanically strong internal shielding system, preferably grounded to the cathode, and also preferably carried on a strong and rigid support extending from the stem. The internal shielding system on this support comprises an internal top shield which, in conjunction with a suitable external shield, is effectively and electrically of a diameter considerably greater than the plate electrode, but which is mechanically a rather deep metal cup, about the diameter of the plate electrode, mounted on the upper end of the support above and coaxial with the electrode system, and two end shields, preferably in the form of metal discs slightly larger than the largest grid and mounted at opposite ends of the grid electrode system to extend transversely of the tube. The support for the shielding system is also a convenient and rigid support for mica or other insulating spacers which accurately and rigidly space the cathode and the grid electrodes surrounding it. The bulb, which is to advantage of the type often referred to as an ST bulb, with a neck and a body portion larger than the neck, is preferably provided at the top or end opposite the neck with a tubular dome or extension of about the same internal diameter as the neck. The cup shaped top shield extends into this dome and usually the tube may be surrounded with an external shield, such as a grounded metal can or cover somewhat larger than the bulb but fitting the dome quite closely adjacent the rim of the top shield. This shielding system is of advantage in a tube having only two grid electrodes, as well as in a tube having three or more grids. Since the maximum diameter of the internal shielding system is substantially that of the plate electrode a mount of which the internal shielding system forms an integral part can be passed thru the neck of the bulb and yet leave ample clearance between the mount and the walls of the body of the bulb.

A tube constructed in accordance with my in vention can be used in many ways and in manifold circuit combinations to great advantage. For example, it is very useful as a biased detector, and as an automatic volume control tube for supplying rectified voltage to the grid of a radio frequency amplifier to affect the bias of the grid to an extent dependent on the strength of the signal voltage impressed on the amplifier. As a radio frequency amplifier it is particularly useful, one important field of use being to gain fidelity control by automatically impressing on the third or regulator grid a voltage which in response to strong signals broadens the resonance curve to the desired extent.

The novel features which I believe to be characteristic of my invention are set out with particularity in the appended claims, but the invention itself, bothas to its organization and method of operation, will best be understood by reference to the following specification taken in connection with the accompanying drawings, which illus trate one form of embodiment of the invention in a tube and some circuits in which some of the properties of the tube are utilized, and in which Figure l is a longitudinal section of one form of tube constructed in accordance with my invention; Figure 2 is a cross section along the line 2--2 of Figure 1; Figure 3 is an enlarged view of part of Figure 2; Figure 4 is a cross section along the line 4-4; Figure 5 is an isometric view of an upper end shield; Figure 6 a side view of the bottom part of the mount, with the bottom end shield and mica spacer in section; and Figure 7 a side view of the middle part of a modified form of control grid.

As best shown in Figure 1 the tube has a sealed envelope such as a bulb 20 which is highly evacuated in the usual way, and which encloses the electrodes. The ,bulb is preferably of the type often referred to as ST, with a maximum diamet r along the section lines 4-4 and tapering to a neck, on which is a base 2| with six contact pins or terminals 22 which for clearness are shown side by side, but which in practice are arranged on the base in a circle. Into the neck of the bulb is sealed 9. reentrant stem 23 of the usual type, on which the electrodes are mounted, and which has the usual press 24 in which the electrode supports and the leading-in wires, which are all made as short as possible, are sealed. The other end or top of the bulb is shaped to form a tubular dome 25 of about the diameterof the bulb neck. A contact cap or terminal 26 is mounted on the dome on the tip of the bulb.

The cathode, of high electron emissivity and preferably of the unipotential indirectly heated type, comprises a heater, such as a reverse wound or double helix 21 of tungsten wire, preferably wound on a refractory insulating core rod, sprayed with a coating of refractory insulation, and supported on the two leads by which it is connected to two of the contact pins 22 forming the heater terminals, and also a tubular cathode sleeve 26, of nickel or similar metal, loosely surrounding the heater. The cathode sleeve, with its walls pinched together at the upper end, has a coating of electron emissive material, such as oxides of barium and strontium. The coating is in two sections 29 which, as best shown in Figure 3, extend lengthwise of the sleeve, but cover only part of the circumference, leaving two diametrically opposite narrow strips or exposed portions 36, of bare nickel between the edges of the coated sections 29.

The first grid electrode or control grid, preferably connected by a grid lead 3| to the cap 26 which is the control .grid terminal, comprises two side rods on which the grid wire 33 is wound into a helix with its turns secured to the side rods. Where the tube is to have a sharp cut off the grid is a helix of uniform pitch; where the tube is to have a variable amplification factor and a comparatively long cut off the grid, as best shown in Figure '7, is a helix of considerably coarser pitch near the middle than at the ends. The control grid may be circular in cross section, but is preferably, as best shown in Figure 3, of substantially circular cross-section'immediately adjacent the cathode, with flattened radial extensions 34 in alignment with the bare strips 30 of the cathode. This form of grid may be made by winding a grid in the usual way, and then forming the radial extensions 34 by flattening or pinching together the grid winding near the side rods 3|. The extensions 34 act as heat radiators or cooling fins to such an extent that the circular portion of the grid may be very close to the cathode without overheating.

The second grid, comprising two side rods 35 on which the grid wire 36 is wound as a circular helix of uniform pitch is connected thru one of its side rods and a lead in the press 24 to the contact pin 22 which is the screen terminal, as usually this second grid is maintained positive to act as a screen grid.

The third or regulator grid comprises two side rods 31 on which the grid wire 38 is wound in a circular helix, one of the side rods being connected thru a lead in the press 24 to the contact pin 22 which is the regulator grid terminal. It is preferably made like the conventional suppressor grid, which is more open and of coarser pitch than the conventional screen grid, and is so proportioned that when it is at cathode popentode with. the usual suppressor grid.

with the grids and the cathode, and carried on side rods 40 in the press .24,,one of'the side rods being electrically connected to the contact pin 22 which is the plate terminal. The anode 39 is preferably carbonized, or otherwise blackened or roughened.

The internal shielding system, mounted on the stem and preferably press 24, comprises a rigid and strong support, such as two comparatively heavy and rigid shield support rods 4| extending from the stem 23 inside the anode and between it and the third grid to the top of the bulb, and preferably in alignment with and in the same diametrical plane as the side rods of the grid electrodes. These rods may be outside the anode,'in which case they may carry shield-.

ing means in addition to what is here shown. One of the shield rods 4| is connected inside the tube to the cathode sleeve 30 and also thru the press to the contact pin 22 which is the cathode terminal. On the upper end of the shield rods 4| a top'shield 42 preferably made somewhat like a flat bottomed deep metal cup with 9. rectangular slot or opening in the bottom, as indicated in Figure 4, is secured to the shield rods 4| by ears 43 welded to the shield rods 4|. The top shield, usually about the same diameter as the anode, so that its bottom covers the ends of the grids ,and the end of the anode, is set so that its sides extend away from the stem and into the dome 25. The top shield is deep enough so that in conjunction with an external shield it 'is effectively of considerably greater electrical low circular dish of approximately the diameter of the largest grid and having in the bottom a narrow diametrical slot with out-turned edges which form in effect two parallel metal plates approximately perpendicular to and extending outwardly from the outside of the bottom of the dish. The top end shield 44 is set over the upper end of the grid system like an inverted dish so that its out turned edges 45 project upwardly thru the slot in the bottom of the top shield 42. These out turned edges 45 of the slot in the bottom of the top end shield extend along and on opposite sides of the upper ends of the cathode and grid rod like parallel plates and greatly reduce inter-electrode electro-static leakage. A similar bottom end shield 46 is placed on the shield rods 4| at the lower end of the grid electrode system, with its out turned edges 41 extending downwardly to form a pair of parallel plates extending alongside and on the opposite sidesof the lower end of the cathode sleeve and of the grid rod. The dish-shaped end shields are preferably set with their rims projecting toward each other and toward the electrode assembly. The entire shielding system is at ground potential, due to a connection between one of the shield rods 4| and the cathode sleeve, and thereby a very complete and efiective shielding and a marked reduction in inter-elecsystem are shielded by end shields, each in the general form of a shal-" into and supported by the dish shaped end shields 44'and 46. Each mica disc has in. registry with the slot in the end shield which supports it a diametrical row of holes into which the ends of the cathode sleeve and of the grid rods fit rather snugly. These mica spacers in the metal end shields, as shown in Fig. l, accurately and rigidly space the grids and the cathode with reference to one another. In the particular construction shown the cathode sleeve and the side rods of the first or control grid electrode are fitted into the corresponding holes in the mica discs so tight that the mica discs carry or support both the cathode sleeve and the first grid, and are in turn supported or carried by the shield rods 4|. The end shields and mica spacers may, if desired, be enlarged to include the anode side rods and mechanically tie the anode to the rest of the electrodes, but usually this is not necessary. The mica discs are so thin that their edges are covered'by the rims of the dish shaped end shields and the discs are also, as best shown in Figure 6, spaced away from the bottom of the dish shaped end shields by inwardly projecting lugs 49 on the bottom of the shields. As a result the presence of the mica discs does not lessen the shielding.

The connection of the cathode sleeve 28 to the bottom end shield 48 and therefore the grounding of the shielding system may conveniently be done, as best shown in Figure 6, by a connector 50 on the cathode sleeve joined to the bottom shield by a strap 5| conveniently formed by making two parallel cuts in the bottom of the shield and bending down the strip of metal between them. A getter tab 52, as shown in Figure 6, and mounted adjacent the press on a wire 53 fixed to a shield rod 4|, has-on the lower side some metal 54 such as magnesium or an alloy of barium and magnesium which can be vaporized, by the use of a high frequency induction coil to perfect the vacuum. The tab isshaped to direct the vaporized metal away from the electrode system and the press, and thereby prevent deposition of metal on the electrodes.

It ,is not necessary, but may be desirable, to secure on the upper end of the shield rods 4| a mica steadying disc 55 which is substantially a mica washer slightly smaller than the internal diameter of the dome 25, to prevent lateral movement of the upper end'of the mount.

The tube is preferably used in conjunction with an external shield 55 which, as shown in Figure 1,

is essentially a metal can or cover somewhat 'conveniently be grounded by mounting it on a grounded metal holder 58 which may be associated with the socket for the base 2| of the tube.

A mount constructed as above described is mechanically strong and rigid, can be made with a high degree of accuracy by means of the usual factory equipment, passed thru the neck of the bulb to the point of maximum diameter of the bulb where the space between the electrodes and the bulb walls is greatest, and sealed in by the usual equipment. The tube shown, with such a mount, has very low plate to ground capacity, good inter-electrode shielding, and very complete electrical separation of the input and output circuits is obtained.

A tube constructed in accordance with my invention can be used in many ways. For example, it may to advantage be used as a biased detector in which case desirable results are obtainable by adjusting the voltage on the third grid to make it either negative, zero or positive. The tube is also useful as an automatic volume control tube in which case it acts as a detector so connected that the radio frequency voltages are applied to the input circuit, and the output circuit is so arranged that a direct voltage which varies at a sub-audio rate appears across the output circuit and is fed back to the radio frequency amplifier stages and also if desired to the first detector stage to control the sensitivity of the receiver. In this case as in the case where the tube is used as a biased detector the ability to adjust the potential of the third grid at will is of advantage.

The tube with the variable pitch grid shown in Figure 7 is particularly useful as a radio frequency amplifier and when so used the third or regulatory grid with its independent and separate terminal permits the attainment of. automatic fidelity control as well as automatic volume control, due in part to the fact that as the third or regulator grid is made more and more negative the plate current shifts more and more to the screen grid, lowering the impedance of the tube, so that the second grid, the third grid and the plate electrode act somewhat like a triode. One explanation of this action is that when the voltage on the third grid is zero, the space charge is almost entirely between the cathode and the first or control grid, but as the voltage of the third or regulator grid becomes more negative the space charge around the second grid inside the third or regulator grid becomes more intense and the tube begins functioning somewhat like a triode. The plate resistance at the operating plate voltage decreases progressively with increasing negative voltage on the third or regulator grid until the third grid reaches some optimum negative voltage after which the plate resistance again rises as the third grid becomes still more negative. Fundamentally this tube is one in which the impedance can be controlled at will by changing the voltage on the third or regulator grid. For example, in a tube embodying the invention and designed to operate at 250 volts on the plate and volts on the second grid or screen, the impedance can be varied from about one million ohms when the third grid is connected to the cathode to about twenty thousand ohms when third grid has a negative bias of about forty (40) volts.

The change of impedance of the tube with change of bias of the third grid permits the tube to be used for automatic fidelity control, since if the plate resistance of a radio frequency amplifier can be made to decrease progressively with increasing strength of. the signal better fidelitycan be obtained for loud signals while retaining good selectivity for the weaker signal. The resonance curve of a simple selective circuit is peaked, and for values of conductance greater than the original the curve becomes flatter in the neighborhood of resonance, indicating that the higher side bands will be transmitted thru the circuit with a greater degree of intensity and consequently the higher audio frequencies will be less attenuated. For loud signals the tube may thus bemade to automatically cause the tuned circuits associated with it to become broadened in their response and consequently cause the fidelity automatically to become better. These results are desirable with the present alignment of transmitter frequency channels, and they may be obtained by varying the plate conductance of the tube with the signal voltage which can be done very nicely with a tube constructed in accordance with my invention by using the third or regulator grid. As the third grid is made more and more negative the plate resistance of the tube automatically goes lower. The plate resistance is in shunt across a simple selective circuit connected to the tube, and as this plate resistance decreases the equivalent series. resistance of the circuit increases, so that the resonance curve is flattened and a better response to the higher frequencies is obtained. For weaker and weaker signals the voltage on the third grid is caused to become less and less negative and the plate resistance of the tube goes higher and higher. The shunt effect on the output circuit becomes smaller and smaller until at substantially zero voltage on the third grid the response of the output circuit is similar to its natural response.

The third grid, particularly when its potential is negative or zero with respect to the cathode, is effective in reducing fluctuation noises, especially those due to secondary emission. In general, making the third grid negative with respect to the cathode tends to reduce the trans conductance of the tube and cause sharp cutofi.

Figure 8 indicates diagrammatically one circuit in which the tube may be used as a biased detector, which in this particular case has an automatic bias connection. In this circuit the input circuit is connected with a control grid 33, the cathode sleeve 28 being connected to an automatic biasing resistor 59 in parallel with a bypass condenser 60, so that the bias is dependent upon the current flowing in the plate circuit. The second grid 36 is maintained sufliciently positive by a battery 6| to act as a screen grid, and the anode is connected to a battery 62. Upon the third or regulator grid 38 is impressed a voltage which may at will be made negative, zero or positive with reference to the cathode and the value of. which depends upon the characteristics desired. This voltage from the third grid may conveniently be derived from a potentiometer comprising a slider 63 cooperating with a resistance 6% connected across the battery 65. The range of the potentiometer may to advantage be from about 40 volts negative to 40 or 50 volts positive. Ordinarily the slider 63 is set at the point which gives the desired characteristics and need not be changed unless conditions change. For a fixed bias detector the automatic biasing resistor and by-pass condenser are omitted and the grid is biased by a battery or similar device. The rectification occurs in the usual way due to the relation between the plate current and control grid voltage.

Figure 9 indicates diagrammatically a. circuit in which the tube is used for automatic volume control. The tube is connected substantially as in Figure 8 except that the automatic biasing resistor and by-pass condenser are omitted and the fixed bias is put on the control grid 33 by a biasing battery 66. The receiving circuit, shown conventionally, comprises a radio frequency amplifier stage of the usual construction, indicated conventionally, and for convenience also indicated diagrammatically as a triode with its grid connected to the input circuit, conventional couplings 61, a detector, an audio amplifier, and a. loud speaker 68. A tube embodying my invention is connected in the circuit to function as an autoaosasce matic volume control tube,- the voltage appearing across the output circuit of the tube being direct voltage at sub-audio frequency which is fed back to the amplifier stages and also if desired to the first detector stage to control the sensitivity of the circuit. In the diagram this direct voltage is fed back to the input of the radio frequency stage thru leads 69 andv a high reactance 10, a resistance II being introduced into the input circuit as shown.

Figure 10 indicates diagrammatically a receiving circuit with the elements indicated conventionally and in which both automatic volume control and fidelity control are obtained by applying to a radio frequency amplifying tube, constructed in accordance with the invention, control voltages derived from the output circuit of the combined detector and automatic volume control unit, which may be of any of the well known types used in the art. In the particular arrangement shown a resistance 12 grounded at one end and connected across the output leads of the detector and automatic volume control unit has cooperating with it two sliders I3, one of which is connected to a lead I4 to the third or regulator grid 38 of the tube and the other thru the lead I5 to the first or control grid 33. By setting the sliders at different points on the resistance different voltages may be impressed on the first and third grids of the tube, and by properly selecting these voltages both automatic volume control and increased fidelity control may be obtained- If desired the leads I5 to the first or control grid can be omitted, and the voltage from the combined detector and automatic volume control unit impressed upon the third regulator grid only. r

Figure 11 shows curves indicating the fidelity control obtained in this tube when used in a suitable circuit. In this figure the curve indicated by It is the resonance curve over a range of 10 kilocycles of a simple selective circuit tuned for maximum selectivity. When a loud signal is received the bias of the third or regulator grid is automatically made more negative by the voltage derived from the combined detector and automatic volume control unit, the extent of this change in bias depending upon the strength of the signal. The resonance curve then changes from that indicated by I6 to that indicated by TI. This change increases the fidelity, reducing the selectivity to some extent at the same time,

but with loud signals the decrease in selectivity is not important. When the signal is so weak that it produces no effect upon the bias of the third grid the selectivity is at a maximum, which is desirable for the reception of weak signals. The net result is that in general the fidelity of reception with a. circuit such as shown in Figme 10 is better than with the usual circuits of this kind.

Figures 12, 13 and'14 show characteristic curves obtained from a tube constructed as above described and operated at about 250 volts on the plate and 100 volts on the second or screen grid.

Figure 12 shows the effect of changes in the When the creased by volt steps, the characteristics change as indicated by the corresponding curves 80, 8|, 82, 83 and 84. It will be seen that while the upper part of the curve I8 has only a slight slope, being substantially fiat, the other curves increase in slope as the negative bias of the regulator grid increases until when the negative bias in th s particular tube is about 40 volts negative the sic-1x: is as indicated by curve 82. As the negative bias further increases the upper part or the curve again begins to flatten, as indicated by curves 83 and 84.

Figure 13 shows the effect upon the plate resistance of this particular tube of changes in the negative bias of the regulator grid for different biases on the control grid. From these curves it is apparent that in this particular tube the plate resistance is a minimum in the neighborhood of 40 volts negative bias on the regulator grid. The useful part of these curves is from zero bias to the biasat which this minimum in plate resistance occurs.

Figure 14 shows the effect on the trans-con ductance of this tube of a change in the voltage on the regulator grid from zero to a negative bias cylindrical anode surrounding and coaxial with said cathode-supports on said stem for said cathode, anode and said grid e1ectrodes,.oi in ternal shielding means comprising a support-extending from said stem and connectedto said cathode, and a cup shaped top shield of approximately the diameter of said anode mounted on the free end'of said support adjacent the ends of said grid electrodes with its bottom extending transversely of said envelope to cover the ends of said grid electrodes and of said anode and with its sides extending away from said stern and into said dome.

2. The combination with an electron discharge device comprising a sealed envelope having at one end a tubular dome and at the other enda re-entrant stem, said envelope enclosing a cathode, three cylindrical grids and a cylindrical anode surrounding and coaxial with said cathode, supports on said stem for said cathode, said grids, and said anode, internal shielding means comprising a support extending from said stem and connected to said cathode, and a cup shaped top shield of approximately the diameter of said anode mounted on the free end of said support adjacent the ends of said electrodes with its bottom extending transversely of said envelope to cover the ends of said electrodes and of said anode and with its sides extending away from said stem, of an external metallic shield enclosing said envelope and having in'one end a hole with its edges adjacent said envelope and in registry with the rim of said top shield.

3. An electron discharge device comprising a stem, a. cylindrical anode supported: on said stem, a cathode coaxial with said anode, three electrically independent cylindrical grids coaxial with said cathode and each comprising a side rod and a wire helix with its turns secured to said side rod, a shield support extending from said stem between said anode and said grids and a connected to said cathode, said shield support and the side rods of said grids being mounted in in the same diametral plane, and an end shield comprising two parallel metal plates mounted on said shield support to extend along and on opposite sides of the ends of said cathode and grid rods.

4. An electron discharge device comprising a stem, a shield support extending from said stem, a cylindrical anode supported on said stem, two end shields mounted on said shield support, each end shield comprising a shallow metal dish with an elongated slot in the bottom, a mica spacer disc fixed in each metal dish and having a row 01' holes in registry with the slot in said dish, a cathode mounted coaxial with said anode and with its ends in the center holes of said mica spacers, and a plurality of grids coaxial with said cathode, each having a side rod with its ends in holes in said mica discs.

5. An electron discharge device comprising a sealed envelope having a re-entrant stem, two shield support rods extending from said stem, two end shields mounted on and spaced apart along said shield support rods, insulating spacers carried by said end shields, a cathode supported by and extending between said insulating spacers, a coaxial grid surrounding said cathode and supported by said insulating spacers, a

plurality of other cylindrical grid electrodes surrounding and coaxial with said cathode and positioned between said end spacers with the ends of all said grids covered by said end shields, a connection between said cathode and said shield support rods, and a cylindrical anode mounted on said stem to surround said grid and said shield support rods.

6. An electron discharge device comprising a stem, a cathode and a coaxial cylindrical grid mounted on said stem, shield rods projecting from said stem, end shield discs adjacent and transverse to the opposite ends of said cathode and grid and secured to said shield support rods and of a diameter greater than said grid, each of said discs having in its. diametrical slot, and mica spacers adjoining and supported by said end shield discs and engaging the ends of said cathode and said grid to position said ends in said slots.

7. An electron discharge device comprising, a cathode and a cylindrical grid arranged coaxially, parallel shield rods extending parallel to said cathode, and metal discs mounted on said shield rods to extend transversely of said cathode adjacent the ends thereof, each of said discs having a diametrical slot with edges turned outwardly, and a mica spacer in each of said end shields for holding the ends of said cathode and of said grids in alignment in said slot and between said out turned edges.

8. An electron dischargedevice comprising an evacuated envelope having a tubular portion at one end and a stem at the other end and enclosing an electrode assembly including a cathode and a coaxial tubular grid, a support rod extending from said stem beyond the ends of said cathode and grid, an insulating spacer intermediate the ends of said support rod for spacing said cathode and grid, and a dished top shield with a cylindrical rim and a flat bottom of greater diameter than said grid mounted on the upper end or said support rod with its bottom concentric with and transverse to said cathode and its rim projecting away from said cathode and ex-- tending into and coaxial with said cylindrical portion of said envelope, and a tubular anode of substantially the diameter of said top shield coaxial with and supported independently of said electrode assembly. I

9. An electron discharge device comprising an evacuated envelope having a stem and enclosing an electrode assembly including a support rod extending from said stem, a dished metal shield with a rim at each end of said assembly, said shields being mounted on said support rod with their rims projecting toward said assembly, sheet mica insulators fitted into said shields with the edges of said insulators covered by the rims of said shields, a thermionic cathode and three tubular grid electrodes coaxially disposed between and carried by said support rods and electrically insulated from one another and from said support rods by said insulators, and a tubular anode coaxial with and supported independently of the remainder of said assembly.

10. An electron discharge device comprising an evacuated envelope having a tubular portion at one end and a stem at the other end, an electrode assembly supported at its lower end on said stem and comprising a cathode, grid, and anode, a cup-shaped top shield mounted on and above the upper end of said assembly with a flat bottom portion over the upper end of said assembly and a cylindrical rim perpendicular to said bottom portion and projecting into and coaxial with said tubular portion of said envelope, and a sheet of mica mounted on the upper edge of the rim of said top shield to contact the walls of said tubular portion and steady the upper end of said assembly in said tubular portion of said envelope.

TERRY M. SHPJADEB. 

